The invention concerns a method of controlling the flow of a liquid in a flow system, the liquid flow comprising particles and being led into a channel thereof, the method comprising the steps of:
enveloping the liquid flow by a flow of carrier liquid,
hydrodynamically focussing the particles in the liquid flow,
providing a measurement signal of the liquid flow from an observation area in the channel, and
dividing the liquid flow at a branching point into two or more outlets in response to said measurement signal.
The sorting of biological materials in the form of cells and microorganisms is typically carried out with a flow-cytometer, which has a sorting module.
A commonly known principle for the sorting of biological materials is brought about by first converting the liquid flow into drops, after which the drops are separated electrostatically.
With another principle, use is made of a method of separation where a volume element in a liquid flow is directed via a separate channel in relation to a main flow.
Examples of the latter principle are e.g. described in U.S. Pat. No. 3,827,555, where the separation is effected by means of mechanical valves, which are controlled on the basis of a signal from a photo-detector. However, the problem with the use of mechanical valves is that they have a relatively poor reaction time, and moreover that they unavoidably influence the flow pattern when they are activated.
In order to alleviate this problem, in U.S. Pat. No. 4,756,427 it is suggested that use be made of a piezo element which, however, is a relatively expensive component.
From WO 98/10267 a flow-switch is known where, by controlling of the part of a right and a left flow, an intermediate liquid flow can be positioned in such a manner that it can be directed through several branches at the outlets. This method is intended especially for the injection of a small liquid volume.
It is now an object of the present invention to provide a method, which results in a faster and cheaper separation of particles.
The object of the invention is achieved by a method of the type disclosed in the preamble to claim 1, which is characterised by introducing a control liquid from at least one control channel at a merging point in the channel, the amount of said control liquid being controlled by at least one electro-kinetic pump, the pump effect of which is controlled in response to said measurement signal.
In this way a system is provided where no use is made of mechanical components, which furthermore makes the system suitable for disposable set-ups, e.g. for use in so-called analysis kits.
The pump control is simple, as the pump effect varies proportionally with the applied electrical field, which is adapted to suit the given characteristics of pump liquid and the dimensions of the pump channels.
It should be noted that in addition to being able to control the central particle flow by the supply of a control liquid, a control of the central particle flow could also be achieved by the electro-kinetic pumping of liquid away from the central flow, naturally providing that this liquid permits the use of electro-kinetic pumping mechanisms. Moreover, the two control methods can be combined, hereby achieving, among other things, a greater displacement of the central particle flow.
It is expedient for the method to be executed as disclosed in claim 2, where said at least one electro-kinetic pump is of an electro-osmotic type consisting of two capillary structures to each of which an electrical field is applied, so that when the field in the one capillary structure is increased, the field in the other capillary structure is correspondingly reduced.
By a particularly simple embodiment of the method according to the invention, as disclosed in claim 4, the amount of control liquid is controlled by just one electro-kinetic pump, which is placed in the one of the channels. By introducing control liquid in greater or smaller degrees, or by pumping liquid away from the channel, the central particle flow can be controlled between the two outlets. The configurations with differences in the channel cross-section, as well as asymmetrical configurations of the branching point, will be able to be used in connection with the controlling of the central particle flow.
Besides, other advantageous configurations of the invention are disclosed in the dependent claims.